The present invention relates to oscillators and, more particularly, to integrated RC oscillator circuits.
Most, if not all, prior art RC oscillators include a comparator having one input coupled to a resistor divider circuit for setting the threshold voltage level of the comparator, a capacitor usually coupled to the other input of the comparator and means for varying the threshold voltage of the comparator by varying the resistance of the divider circuit.
Typically, a PNP transistor is coupled to the output of the comparator which is switched between off and on states to change the threshold voltage level. In the off state a upper threshold voltage is established at the one input of the comparator, across the resistor divider, and a current source supplies current to charge the capacitor until the voltage thereacross reaches the upper threshold voltage. At this time the output of the comparator switches which turns on the PNP transistor. As the PNP transistor is switched on the resistance of the resistor divider is decreased, usually by shorting out a portion of the lower resistance of the divider, thereby establishing a lower threshold voltage at the one input of the comparator. The capacitor is then allowed to discharge to this lower threshold voltage at which time the PNP transistor is switched off as the output of the comparator is switched to its previous state. In this manner the capacitor is alternately charged and discharged. The output of the oscillator is usually provided from across the capacitor as is well understood.
A problem with the above described prior art oscillators is due to the fact that PNP transistors have notoriously slow switching characteristics. This characteristic limits the cycle time for charging and discharging the capacitor and, hence, the oscillation frequency of the oscillator.
Hence, a need exists for an RC type of oscillator that does not require a PNP switching transistor in order to provide high frequency oscillation signals.